Such light sources are known per se. They are used, inter alia, in luminaires for general lighting purposes, for example, for office lights, for shop lights or, for example, for shop window lights. These light sources are also used, inter alia, in special lighting applications such as hospital operating illumination units, for example, used during surgery, or special lighting applications used, for example, in graphical industry, printing industry, or specific shop light applications and advertisements.
A current trend in light sources is to improve the efficiency of the light source, for example, by using semiconductor devices as light emitting devices. These semiconductor light emitting devices are, for example, Light Emitting Diodes (further also indicated as LED) or semiconductor LASERs. Recently also Organic LEDs (also further indicated as OLED) are often used. These semiconductor light emitting devices typically emit light within a limited spectral bandwidth at relatively high intensity. To ensure that these light sources comprising semiconductor light emitting devices still have decent color rendering, often luminescent materials are included in the light source. In a luminescent material, light of a first spectral distribution is absorbed and is partially converted into light of a second spectral distribution. Typically the absorbed photons are emitted as photons having a lower frequency (or longer wavelength: Stokes shift) and the loss of energy per photon is transformed into heat. Alternatively, the emitted photons may have higher frequency (or shorter wavelength: anti-Stoke shift).
One example of such light source comprising a luminescent material can be found in the published patent application US2010/0123155A1 which relates to a semiconductor nanoparticle-based light-emitting device. This device comprises a LED that is arranged to emit blue primary light upon the application of current and which is submerged in a commercially available LED encapsulant in which is embedded a plurality of quantum dot-containing polymeric beads. Although the known light sources usually have a relatively good color rendering, often contrast between colors is not good enough.